Vehicles having an internal combustion engine can operate in a variety of modes. As one example, an engine may operate in a spark ignition (SI) mode, wherein a charge of a mixture of air and fuel is ignited by a spark performed by a sparking device within a combustion chamber. As another example, an engine may operate in a compression ignition mode, wherein a mixture of air and fuel are compressed within a combustion chamber by a piston, causing ignition of the charge without necessarily requiring the addition of a spark from a sparking device.
One type of compression ignition known as homogeneous charge compression ignition (HCCI) utilizes compression of a substantially homogeneous mixture of air and fuel to achieve controlled auto-ignition (CAI). In HCCI engines, ignition occurs virtually simultaneously throughout a combustion chamber as a result of compression instead of spark ignition, making the combustion process challenging to control. HCCI engines are similar to conventional gasoline engines in having a homogeneous charge, but are similar to conventional diesel engines in having compression ignition. HCCI engines may be used to combine gasoline engine low emissions with diesel engine efficiency.
Unfortunately, HCCI combustion engines typically change operation conditions more slowly than other combustion processes. The engine hardware used to control initial cylinder conditions such as internal residuals, intake air temperatures, and the combustion process stability window, limits dynamic response.
Engine control strategies have been developed that switch between combustion modes to increase dynamic response of HCCI engines. In one approach, as described in U.S. Patent Application Publication 2004/0182359, issued to Stewart, et al., individual cylinders may be switched between combustion modes to allow use of HCCI combustion over a wider load and speed range. In another approach, as described in U.S. Pat. No. 6,390,054, issued to Yang, two different groups of cylinders may be transitioned between combustion modes in different stages to allow more control over dynamic response.
However, the inventors herein have recognized disadvantages with these approaches. Specifically, by switching combustion modes the beneficial aspects of HCCI combustion are only enjoyed over a limited range of operating conditions. Additionally, in HCCI engines, as engine load increases ignition typically advances and changes combustion rates, thermal efficiency and harshness, yet while engine load decreases, ignition typically retards and can result in misfiring and increased emissions. Also, HCCI combustion typically generates low emissions of nitrogen oxides (NOx) due mainly to lower combustion temperatures, but it can generate relatively high hydrocarbon (HC) and carbon monoxide (CO) emissions and can generate higher levels of NOx emissions at higher engine speeds and larger engine loads. Additionally, lean burn engines typically operate outside of the optimal range of operation for typical emissions control devices such as 3-way catalytic converters and may require use of expensive emissions control devices such as NOx trap catalytic converters.
The inventors herein have recognized the above-mentioned disadvantages and have developed a system that improves operating ranges and emission control for HCCI combustion engines.